1. N.D. Pullen, British Patent 49, (17 December, 1934), p 162.
2. R.B. Mason, M. Tosterud, US Patent 2040618, 12 May, (1936); 2108(1938) pp. 603.
3. T. Nakayama, Japenese Patent 12889, (1939).
4. S. Wernick, Pinner, The Surface Treatment and Finishing of Al and its Alloys, third ed., Robert Droper, Teddingaton, 1964.